Stabilized uncured copolymers of butadiene and styrene



United States Patent ()1 3,057,820 STABILIZED UNCURED COPOLYMERS FBUTADIENE AND STYRENE Richard W. Bell, Ernest Csendes, and Herbert F.Mc-

. Shane, Jr., Wilmington, DeL, assignors to E. I. du Pont de Nemours andCompany, Wilmington, Del., a corporation of Delaware No Drawing. FiledJune 9, 1959, Ser. No. 819,012

1 Claim. (Cl. 260-4585) This invention is directed to a new compositionof matter, an SBR rubber protected against gel formation duringmechanical shearing at elevated temperatures.

SBR is a synthetic rubber which is a copolymer of butadiene and styreneand has become an increasingly important article of commerce in recentyears. Unfortunately, the uncured product is very sensitive to heat.After it has been made by emulsion polymerization, it is isolated as wetcrumbs which are dried by passage through a heated zone. Unless astablizer is present, serious resinification may occur during thisoperation. It has, accordingly, been the customary practice to introducea dispersion of a water-insoluble antioxidant into the emulsion beforethe crumbs are isolated. When dry;i the stabilized crumb is shipped toprocessors in the tra c.

There it is compounded and cured to give the desired elastomericarticles. While being compounded, the uncured SBR rubber stock issubjected to considerable shearing action on rollers or in Banburymixers. This shearing action generates heat Which raises the temperatureof the uncured SBR rubber close to 150 C. In spite of the stabilizerpresent, the character of the SBR rubber then begins to change andsatisfactory processing becomes steadily more difficult. Within 20minutes at 150 C. the gel content of the SBR rubber stock may rise from0% to a value as high as 50%. This change is reflected by an observedincrease in the values of the Mooney viscosity as well as the Williamsplasticity and recovery. The faster the SBR rubber is processed, thehigher the processing temperature will rise and the more pronounced thisgel formation Will become. This gel formation is highly undesirable. Itis accompanied by a temporary increase in the Mooney viscosity; thisirregularity makes it very difiicult to process such stock in areproducible fashion. Equally important, when gel is present, inferiorvulcanizates are obtained: they display a higher modulus, break atshorter extensions and display much less resistance to flex crackingthan do the vulcanizates made from gel-free stock.

It is an object of the present invention to provide a stabilized uncuredhot SBR rubber not heretofore attainable in the prior art. It is anotherobject of this invention to provide a stabilized uncured SBR rubbercomposition which composition is stabilized against gel formation duringmechanical shearing at elevated temperatures. It is a further object ofthis invention to stabilize uncured SBR rubber against gel formationduring milling at temperatures above 135 C.

These and other objects will become apparent in the followingdescription and claims.

More specifically, the present invention is directed to an uncuredcopolymer of 1,3-butadiene and styrene stabilized against gel formationduring mechanical shearing above about 135 C. by incorporating therein(1) at least 0.75% by weight of a phenolic antioxidant, and, (2) atleast 0.1% by weight of a promoter compound selected from the groupconsisting of (A) polyols having the structure HOCH CHOH) CH X,

Where X=OH, NH(lower alkyl); (B) ammonium citrates; (C) acetate salts ofN(lower alkyl)glucamines.

3,057,820 Patented Oct. 9, 1962 Ammonium citrates which may be utilizedhave the structure where R=H, an alkyl radical, a cycloalkyl radical, ahydroxyl-substituted alkyl radical, a. polyhydroxy-substituted alkylradical, an aryl radical, or an aralkyl radical; R, R", R (which neednot be the same) :H, an alkyl radical, a cyloalkyl radical, ahydroxyl-substituted alkyl radical, a polyhydroxy-substituted alkylradical, or an aralkyl radical; with the proviso that R and R may bejoined to form a polymethylene radical having 3 to 7 chain C-atoms or a3-oxapentamethylene radical; X: citrate anion. Additional operableammonium citrates include citrate salts of the fol-lowing unsaturatedheterocyclic bases: pyridine; pyridine having at least one C-atombearing an alkyl, hydroxyalkyl, polyhydro-xyalkyl, alkoxy, aryloxy, orhalogen radical; quinoline; quinoline having at lea-st one C-atombearing an alkyl, hydroxyalkyl, polyhydroxyalkyl, alkoxy, aryloxy, orhalogen radical; isoquinoline; isoquinoline having at least one C-atombearing an alkyl, hydroxyalkyl, polyhydroxyalkyl, alkoxy, aryloxy, orhalogen radical.

The SBR rubber used in this invention contains about 4 to 43% styreneunits by weight of the copolymer. The preferred copolymer, whichincorporates about 23.5% styrene units by weight, exhibits an intrinsicviscosity in toluene of about 2.09 which corresponds to a viscosityaverage molecular weight of about 270,000; its transition temperature isabout 62 C.; its density at 25 C. is about 0.93 g. per cc.; the Mooneyviscosity of raw uncompounded polymer read after 4 minutes operation ofthe large rotor at C. ranges between about 46 to 54.

The antioxidant and the promoter are introduced, according to thisinvention, into uncured SBR before it is subjected to mechanicalshearing at temperatures above C. In order to prevent gel formation, itis preferred to introduce all of the promoter before the uncured SBRrubber stock reaches a temperature of 135 C. As has been pointed outheretofore, in general commercial practice it is necessary to introducethe antioxidant into SBR latex in order to protect the SBR polymercrumbs during the subsequent drying stage. However, the antioxidant maybe introduced into the gel-free solid polymer itself.

Since the promoters are water-soluble compounds (or compounds wihich arequite'hydrophilic), they will be lost (at least inpart) it added to anaqueous latex unless special techniques are used to protect them;ordinarily, therefore, the promoters would not be added to an aqueouslatex. Accordingly, the promoter is preferredly introduced into theuncured SBR rubber stock at the time it is compounded (unless specialtechniques are employed during the isolation of the polymer crumbs fromthe latex). It should be noted that the copolymer crumbs receive addedprotection during the drying stage when the promoter is present. Thepromoter may be added to the latex by special techniques to accomplishthis result, when desired. For example, the promoter is encapsulated inan oil-soluble material before it is added.

It is believed that any phenolic antioxidant may be used in thisinvention. By phenolic antioxidant is meant a phenolic compound, which,when added to elastomers in small proportions, retards atmosphericoxidation or the effect of oxidation.

3 Representative examples of antioxidants which may be employed are:

2,2'-methylenebis 6-tert-butyl-4-rnethyl phenol),

2,2-methylenebis(6-tert-butyl-4-ethyl phenol),

2,2-methylenebis [4-methyl-6-(1,1,3,3-tetramethyl)butyl phenol],

4,4'-bis Z-tert-butyl-S-methyl phenol) sulfide,

4,4'-butylidene-bis(2-tert-butyl-5-methyl phenol),

2,2-methylenebis(4,6-dimethyl phenol),

2-tert-butyl-4 4-tert-butyl phenyl phenol,

2-tert-butyl-4-phenyl phenol,

2,6-dibenzyl-4-methyl phenol,

2-benzyl-4-methyl phenol,

2-benzyl-6-tert-butyl-4-methyl phenol,

2-benzyl-6-tert-butyl-4-ethyl phenol,

2,4-dimethyl-6-(l-methyl-l-cyclohexyl) phenol,

2,6-diisopropyl-4-methyl phenol,

2,4-dimethyl-6-isopropyl phenol, 2-tert-butyl-4,6-dimethyl phenol,

2-tert-butyl-4-methyl phenol,

2-( l,l,3,3-tetramethyl butyl)-4-methyl phenol,

2,4,6-trimethyl phenol,

2,6-di-tert-butyl-4-methyl phenol,

2,6di-tert-butyl-4-ethyl phenol,

4-phenyl phenol,

2,6-diisopropyl phenol,

2,6-di-tert-butyl-4-phenyl phenol,

2,6-di-tert-butyl-4-(4-tert-butyl-phenyl) phenol,

2,5-di-tert-butyl-hydroquinone,

2,5-di-tert-amyl-hydroquinone,

and alpha-conidendrine. Mixtures of the foregoing may be used.

There appears to be no critically in the ratio of the promoter compoundto the antioxidant; however, better results are obtained when at leastabout 0.20 part of the promoter compound is present for each part bweight of the antioxidant. In practice, the concentrations (by weight ofthe SBR rubber) of the promoter and the antioxidant may range,respectively, from about 0.1 to and about 0.75 to 1.75%. Lesssatisfactory protection is gained when lower concentrations are used;higher concentrations are less economically attractive. The preferredconcentrations of the promoter and the antioxidant range, respectively,from about 0.3 to 1.5% and about 1.0 to 1.5

Representative examples of polyols which are operable as promoters inthis invention are the following reduced sugars: sorbitol, mannitol,galactitol, talitol, iditol, allitol, altritol, and gulitolRepresentative examples of operable N-alkyl substituted polyols are thefollowing glucamines: N-methyl glucamine, N-ethyl glucamine, and N-butylglucamine. Representative examples of the citrates having the structure(RRR"R"N) +Y include the following operable compounds: diammoniumcitrate, N-ethyl glucamine monohydrogen citrate, N-butyl glucaminemonohydrogen citrate, butylamine monohydrogen citrate, isooctylaminemonohydrogen citrate, octadecylamine monohydrogen citrate, diethylaminemonohydrogen citrate, pyrrolidine monohydrogen citrate, morpholinemonohydrogen citrate, 2-diethylaminoethanol monohydrogen citrate,diethanolamine monohydrogen citrate, triethanolamine monohydrogencitrate, benzyltrimethylammonium monohydrogen citrate, dicyclohexylaminemonohydrogen citrate, aniline monohydrogen citrate, N-methylanilinemonohydrogen citrate, p-toluidine monohydrogen citrate, m-anisidinemonohydrogen citrate and o-chloroaniline monohydrogen citrate.Representative examples of the acetates of N-(lower alkyl) glucaminesare: N-methyl glucamine acetate; N-ethyl glucamine acetate; N-isobutylglucamine acetate. Representative examples of citrates of unsaturatedheterocyclic bases are: pyridine monohydrogen citrate, beta-picolinemonohydrogen citrate, quinoline monohydrogen citrate, quinaldinemonohydrogen citrate, isoquinoline monohydrogen citrate, and4-methoxyisoquinoline monohydrogen citrate.

In the following representative examples there may be substituted any ofthe aforedesignated specific antioxidants with any of the promotercompounds identified to give essentially the same results.

A representative example illustrating the present invention follows.

The Mooney viscosities are obtained by the procedure of ASTM MethodD927. A large rotor is used. The sample is sheared for 4 minutes at 212F. C.).

The gel content of the SBR is determined by a method based on the factthat the gel portion of SBR is insoluble in benzene The sample isextracted with benzene, the insoluble gel is filtered off on a 100-meshscreen, and the sol determined by evaporating an aliquot of the filtrateto dryness. The gel is determined by the difference. The apparatusutilized and procedure followed is as follows:

APPARATUS UTILIZED 1) Small-neck, screw-cap, 4-oz. brown glass bottles.The caps shall contain liners of tin, aluminum, or other material notattacked by benzene.

(2) Roller on which to roll the bottles.

(3) Pipettes (bulb type)-25 and 100 cc. capacity.

(4) Beakers50 cc. capacity.

(5) Hot plate (steam or electric).

(6) 100 C. oven.

(7) Wire screen cylinder, closed at one end, to fit friction-tight inbottle. The screen shall be 100 mesh and made from stainless steel.

(8) Analytical balance.

PROCEDURE FOLLOWED Weigh accurately on the analytical balance, 0.4-0.5grams of the SBR sample, not over 0.025" thick, into a 4-oz. bottle andpipette into the bottle 100 cc. of dry thiophene-free benzene. Thebenzene shall be stored in a constant temperature room so that thespecific gravity will remain constant from day to doy.

Turn the bottle on the roller for 48:2 hours. Remove the bottle from theroller and store undisturbed in a constant temperature room until thesolution temperaure reaches an equilibrium. Two hours is usuallysufficient.

Leave the bottle undisturbed thereafter until any fine gel particleswhich may be present have settled to the bottom.

Then uncap the bottle and carefully insert the wire screen into thesolution. Do not lower it in the bottle farther than necessary to drawout the aliquot sample. This will prevent undue mixing of the gelparticles with the solution.

Pipette out a 25-cc. aliquot, using gentle vacuum to minimize stirringof the solution, into a 50-cc. beaker which has previously been heatedfor 30 minutes in the 100 C. oven, cooled in a calcium chloridedesiccator, and weighed. Evaporate the solution to dryness. Care istaken not to overheat the dried polymer as it will start to discolor.

Dry the beaker to constant weight in the 100 C. oven for 30 minutes,cool in the desiccator, and weigh.

CALCULATION OF GEL Wt. of sample-4X wt. of residue P ercent gel Weightof sample Example TABLE 1,-GEL INHIBITION OF SBR CONTAINING PHENO- LIOAN TIOXIDANTS Milling Mooney Gel Antioxidant Additive Time ViscosityContent,

Percent Styrenated phenol. None 10 61 34 Do Diethanolamine 5 53monohydrogen 1O 69 17 citrate. 20 74 44 Do Sorbitol 47 5 A mixture oftert-butyl and tert-octyl substituted mand p-cresols (commerciallyavailable as Wingstay T) is substituted for a styrenated phenol in theprocedures of parts A, B and C. The results are given in Table 2.

In the preceding example 2,6-di-tertiary-butyl-4-phenylphenol may besubstituted to obtain essentially the same results.

The representative SBR rubber utilized in the preceding example contains23.5% by weight of styrene units.

It is to be understood that any of the heretofore described promoters,antioxidants and SBR rubbers may be utilized in accordance with thepreceding specific example to give substantially the same results.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that this invention is not limited to the specificembodiments thereof except as defined in the appended claim.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

An uncured copolymer of 1,3-butadiene and styrene stabilized against gelformation during mechanical shearing at a temperature above about C. byhaving incorporated therein (1) at least 0.75% by weight of saidcopolymer of a phenolic antioxidant, and, (2) at least 0.1%, by weightof said copolymer of a promoter compound selected from the groupconsisting of (a) a polyol having the structure HOCH (CHOH) CH X,wherein X is a lower alkyl --NH, (b) an ammonium citrate, and, (c) anacetate salt of an N(l0wer alkyl)glucarnine, said alkyl group having 1to 4 carbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS2,343,950 Berne-Allen Mar. 14, 1944 2,721,185 Schulze et a1 Oct. 18,1955 2,734,881 Lally et al. Feb. 14, 1956 2,999,841 Osendes Sept. 12,1961

